Although the molecular biology of Salmonella typhimurium is rather well understood, it is only recently that significant progress has been made in the identification of specific traits and in the delineation of cues and regulatory mechanisms for these virulence traits. The expression of specific virulence traits has been shown to be in response to contact with host cells, phagocytosis by macrophages, osmolarity, to contact with host cells, phosphate concentrations, and temperature. Since the thermoregulation of virulence traits is well established in other pathogenic Enterobacteriaceae, it is reasonable to assume that S. typhimurium may exhibit similar broad responses. It is proposed to identify genes that exhibit temperature-dependent expression and to use these genes to locate the regulatory elements controlling their temperature dependent expression. Specifically, TnphoA insertional mutagenesis will be used to identify and clone periplasmic space and outer membrane protein associated genes that are thermoregulated with particular reference to determinants affecting serum-resistance. The insertional locus will be characterized by the size of the XbaI restriction fragment in which the insertion has occurred and the relative location and orientation within that fragment. The gene product will be identified by the electrophoretic mobility of the protein altered by the insertion. A potential role in virulence will be assessed by the effect of the insertion on serum-resistance and in mouse virulence assays. The cloned mutations will provide probes for the isolation of the wild-type genes. The phoA: fusions will also serve as reporter genes in the Mu d2-8 mediated generation of lac fusions in the identification of regulatory elements controlling temperature-dependence.